Basic concepts (mod 4 linked) Flashcards
What is Enthalpy? State the SI unit for enthalpy and its symbol
Enthalpy is the measure of potential energy stored in chemical bonds.
The SI unit for enthalpy is KILOJOULES PER MOLET.
(Chemistry uses kilojoules as the norm, therefore be careful when converting values in the Q = MCΔT)
The symbol for Enthalpy is H and therefore when finding the change in enthalpy, it is expressed through ΔH
What does the universe favor for enthalpy? What is the natural tendency?
The universe favours minimum enthalpy, meaning that it tends to favour less energy present.
Thus, the universe favours exothermic reactions, as energy is released in the forward direction towards the product side and therefore, less energy is available afterwards.
There is a natural tendency for enthalpy to decrease (Don’t have to explicitly describe the direction forward or reverse for this one)
In an exothermic reaction, which side of the reaction is favored? The product or reactant side? Why?
The product side is favored, since exothermic reactions release energy. This means that the product side of the equation would have a minimum enthalpy. The universe favors whichever side has minimum enthalpy and in an exothermic reaction, it is evidently the product.
Therefore, the product side of an exothermic reaction is favored
Which side of the chemical equation is favoured by the universe in an endothermic reaction? Why?
In an endothermic reaction, the reactant side is favoured. This is because energy is absorbed, meaning that energy is lower on the reactant side and greater on the product side. The product side is the final form of the initial, meaning that it is the initial reactant side with additional energy. Therefore, the universe favours the side with minimum enthalpy, which is less energy.
The reactant side has less energy compared to the product side in an endothermic chemical equation.
Therefore the reactant side is favoured.
What is entropy? What are the SI units and symbols for entropy? What is the order of entropy increase regarding the states of matter?
Entropy is the measure of disorder and chaos within a system. A change in entropy is expressed through ΔS and the SI unit for entropy is Kilojoules per Kelvin.
The order of entropy increase is in the following order:
Solids –> Liquids –> Aqueous –> Gases
What does the universe favour for entropy?
The universe favours maximum entropy.
This also means that the universe favours an increase in entropy, meaning there is a tendency for increased chaos and disorder/randomness.
If both sides of a chemical equation have gases, then how can you tell the direction of entropy and if it is increasing or decreasing?
Entropy is increasing or at its maximum on the side where there is a greater number of moles of gas molecules.
Therefore, don’t count the amount of particles, rather the amount of moles on each side and see which one has more.
Greater number of moles of gaseous particles mean entropy is at its maximum and thus increasing.
Under what circumstances would no chemical reactions occur? (Hint, reference and explain using natural tendencies and what the universe favours)
The universe favours minimum enthalpy and maximum entropy.
As such, if a reaction had minimum enthalpy on the reactant side and had maximum entropy also in the reactant side, then no reaction will ever occur. This is because the reactant side has already achieved what the universe wants.
Therefore, there will be no reaction if the entropy is at its maximum and enthalpy at its minimum - both on the reactant side of the chemical equation.
When will a reaction proceed until completion?
When the reactant side of a chemical equation does not have either of the universal tendencies, then the reaction will do whatever it takes in order to fall into the favour of the universe.
Therefore, in order to reach maximum entropy and minimum enthalpy (which the universe favours) it must have minimum entropy and maximum enthalpy on the reactant side
Therefore, the product side would have to favour the universe, meaning that there is maximum entropy and minimum enthalpy in the product side.
- Minimum enthalpy and maximum entropy favours the products of the chemical reaction
What does the ‘heat term’ or ‘energy term’ imply? (E.g. +46 kJ in a chemical equation)
The heat term or energy term simply means energy
However, whichever side the heat term is in, that side holds minimum enthalpy.
Therefore, if the symbol/value was on the reactant side (left hand side), the reactants would have minimum enthalpy, thus making it an endothermic reaction
But if the heat term was on the product side, this side would now have minimum enthalpy and thus, the reaction favours a decreasing enthalpy in the forward direction.
What do we call the two main directions, when talking about what is happening to the entropy or enthalpy in a chemical equation?
Entropy and Enthalpy is either increasing or decreasing in the forward direction or the reverse direction.
Therefore, ‘Forward’ or ‘Reverse’
When is a reaction exothermic? Refer to enthalpy
A reaction is exothermic when the enthalpy is less than zero.
ΔH < 0
(This means energy has been released into the atmosphere in the form of different types of energy, specifically heat)
When is a reaction endothermic, with reference to enthalpy
When ΔH > 0
Since energy has been absorbed into the system
what happens when entropy is greater than zero? ΔS > 0
If ΔS > 0, the system is getting more chaotic
What happens when entropy is less than zero? ΔS < 0
If ΔS < 0, then entropy is decreasing within the system
